This invention relates to the field of isolation systems for use in selectively isolating electrical circuits from one another. More particularly, this invention relates to digital isolation systems including analog-to-digital converter (ADC) offset calibration systems.
Electrical isolation barriers can be identified in many industrial, medical and communication applications where it is necessary to electrically isolate one section of electronic circuitry from another electronic section. In this context isolation exists between two sections of electronic circuitry if a large magnitude voltage source, typically on the order of one thousand volts or more, connected between any two circuit nodes separated by the barrier causes less than a minimal amount of current flow, typically on the order of ten milliamperes or less, through the voltage source. An electrical isolation barrier must exist, for example, in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. Specifically, in order to achieve regulatory compliance with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
In many applications there exists an analog or continuous time varying signal on one side of the isolation barrier, and the information contained in that signal must be communicated across the isolation barrier. For example, common telephone network modulator/demodulator, or modem, circuitry powered by a residential wall outlet must typically transfer an analog signal with bandwidth of approximately 4 kilohertz across an isolation barrier for transmission over the two-wire, public switched telephone network. The isolation method and associated circuitry must provide this communication reliably and inexpensively. In this context, the transfer of information across the isolation barrier is considered reliable only if all of the following conditions apply: the isolating elements themselves do not significantly distort the signal information, the communication is substantially insensitive to or undisturbed by voltage signals and impedances that exist between the isolated circuitry sections and, finally, the communication is substantially insensitive to or undisturbed by noise sources in physical proximity to the isolating elements.
High voltage isolation barriers are commonly implemented by using magnetic fields, electric fields, or light. The corresponding signal communication elements are transformers, capacitors and opto-isolators. Transformers can provide high voltage isolation between primary and secondary windings, and also provide a high degree of rejection of lower voltage signals that exist across the barrier, since these signals appear as common mode in transformer isolated circuit applications. For these reasons, transformers have been commonly used to interface modem circuitry to the standard, two-wire telephone network. In modem circuitry, the signal transferred across the barrier is typically analog in nature, and signal communication across the barrier is supported in both directions by a single transformer. However, analog signal communication through a transformer is subject to low frequency bandwidth limitations, as well as distortion caused by core nonlinearities. Further disadvantages of transformers are their size, weight and cost.
The distortion performance of transformer coupling can be improved while reducing the size and weight concerns by using smaller pulse transformers to transfer a digitally encoded version of the analog information signal across the isolation barrier, as disclosed in U.S. Pat. No. 5,369,666, xe2x80x9cMODEM WITH DIGITAL ISOLATIONxe2x80x9d (incorporated herein by reference). However, two separate pulse transformers are disclosed for bidirectional communication with this technique, resulting in a cost disadvantage. Another disadvantage of transformer coupling is that additional isolation elements, such as relays and opto-isolators, are typically required to transfer control signal information, such as phone line hookswitch control and ring detect, across the isolation barrier, further increasing the cost and size of transformer-based isolation solutions.
Because of their lower cost, high voltage capacitors have also been commonly used for signal transfer in isolation system circuitry. Typically, the baseband or low frequency analog signal to be communicated across the isolation barrier is modulated to a higher frequency, where the capacitive isolation elements are more conductive. The receiving circuitry on the other side of the barrier demodulates the signal to recover the lower bandwidth signal of interest. For example, U.S. Pat. No. 5,500,895, xe2x80x9cTELEPHONE ISOLATION DEVICExe2x80x9d (incorporated herein by reference) discloses a switching modulation scheme applied directly to the analog information signal for transmission across a capacitive isolation barrier. Similar switching circuitry on the receiving end of the barrier demodulates the signal to recover the analog information. The disadvantage of this technique is that the analog communication, although differential, is not robust. Mismatches in the differential components allow noise signals, which can capacitively couple into the isolation barrier, to easily corrupt both the amplitude and timing (or phase) of the analog modulated signal, resulting in unreliable communication across the barrier. Even with perfectly matched components, noise signals can couple preferentially into one side of the differential communication channel. This scheme also requires separate isolation components for control signals, such as hookswitch control and ring detect, which increase the cost and complexity of the solution.
The amplitude corruption concern can be eliminated by other modulation schemes, such as U.S. Pat. No. 4,292,595, xe2x80x9cCAPACITANCE COUPLED ISOLATION AMPLIFIER AND METHOD,xe2x80x9d which discloses a pulse width modulation scheme; U.S. Pat. No. 4,835,486 xe2x80x9cISOLATION AMPLIFIER WITH PRECISE TIMING OF SIGNALS COUPLED ACROSS ISOLATION BARRIER,xe2x80x9d which discloses a voltage-to-frequency modulation scheme; and U.S. Pat. No. 4,843,339 xe2x80x9cISOLATION AMPLIFIER INCLUDING PRECISION VOLTAGE-TO-DUTY CYCLE CONVERTER AND LOW RIPPLE, HIGH BANDWIDTH CHARGE BALANCE DEMODULATOR,xe2x80x9d which discloses a voltage-to-duty cycle modulation scheme. (All of the above-referenced patents are incorporated herein by reference.) In these modulation schemes, the amplitude of the modulated signal carries no information and corruption of its value by noise does not interfere with accurate reception. Instead, the signal information to be communicated across the isolation barrier is encoded into voltage transitions that occur at precise moments in time. Because of this required timing precision, these modulation schemes remain analog in nature. Furthermore, since capacitively coupled noise can cause timing (or phase) errors of voltage transitions in addition to amplitude errors, these modulation schemes remain sensitive to noise interference at the isolation barrier.
Another method for communicating an analog information signal across an isolation barrier is described in the Silicon Systems, Inc. data sheet for product number SS173D2950. (See related U.S. Pat. No. 5,500,894 for xe2x80x9cTELEPHONE LINE INTERFACE WITH AC AND DC TRANSCONDUCTANCE LOOPSxe2x80x9d and U.S. Pat. No. 5,602,912 for xe2x80x9cTELEPHONE HYBRID CIRCUITxe2x80x9d, both of which are incorporated herein by reference.) In this modem chipset, an analog signal with information to be communicated across an isolation barrier is converted to a digital format, with the amplitude of the digital signal restricted to standard digital logic levels. The digital signal is transmitted across the barrier by means of two, separate high voltage isolation capacitors. One capacitor is used to transfer the digital signal logic levels, while a separate capacitor is used to transmit a clock or timing synchronization signal across the barrier. The clock signal is used on the receiving side of the barrier as a timebase for analog signal recovery, and therefore requires a timing precision similar to that required by the analog modulation schemes. Consequently one disadvantage of this approach is that noise capacitively coupled at the isolation barrier can cause clock signal timing errors known as jitter, which corrupts the recovered analog signal and results in unreliable communication across the isolation barrier. Reliable signal communication is further compromised by the sensitivity of the single ended signal transfer to voltages that exist between the isolated circuit sections. Further disadvantages of the method described in this data sheet are the extra costs and board space associated with other required isolating elements, including a separate high voltage isolation capacitor for the clock signal, another separate isolation capacitor for bidirectional communication, and opto-isolators and relays for communicating control information across the isolation barrier.
Opto-isolators are also commonly used for transferring information across a high voltage isolation barrier. Signal information is typically quantized to two levels, corresponding to an xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d state for the light emitting diode (LED) inside the opto-isolator. U.S. Pat. No. 5,287,107 xe2x80x9cOPTICAL ISOLATION AMPLIFIER WITH SIGMA-DELTA MODULATIONxe2x80x9d (incorporated herein by reference) discloses a delta-sigma modulation scheme for two-level quantization of a baseband or low frequency signal, and subsequent communication across an isolation barrier through opto-isolators. Decoder and analog filtering circuits recover the baseband signal on the receiving side of the isolation barrier. As described, the modulation scheme encodes the signal information into on/off transitions of the LED at precise moments in time, thereby becoming susceptible to the same jitter (transition timing) sensitivity as the capacitive isolation amplifier modulation schemes.
Another example of signal transmission across an optical isolation barrier is disclosed in U.S. Pat. No. 4,901,275 xe2x80x9cANALOG DATA ACQUISITION APPARATUS AND METHOD PROVIDED WITH ELECTRO-OPTICAL ISOLATIONxe2x80x9d (incorporated herein by reference). In this disclosure, an analog-to-digital converter, or ADC, is used to convert several, multiplexed analog channels into digital format for transmission to a digital system. Optoisolators are used to isolate the ADC from electrical noise generated in the digital system. Serial data transmission across the isolation barrier is synchronized by a clock signal that is passed through a separate opto-isolator. The ADC timebase or clock, however, is either generated on the analog side of the barrier or triggered by a software event on the digital side of the barrier. In either case, no mechanism is provided for jitter insensitive communication of the ADC clock, which is required for reliable signal reconstruction, across the isolation barrier. Some further disadvantages of optical isolation are that opto-isolators are typically more expensive than high voltage isolation capacitors, and they are unidirectional in nature, thereby requiring a plurality of opto-isolators to implement bidirectional communication.
It is recognized by those skilled in the art that ADC""s, such as may be useful in digital isolation systems, may produce inherent offset signals due to minor differences in component geometries and process variations. That is, if a xe2x80x9czeroxe2x80x9d signal is provided at the input of an ADC, a non-zero output may be generated by the ADC. The amount of offset will vary from one ADC to another, because the offset is often due to random variations in the devices. It is desirable to provide an efficient mechanism for calibrating the isolation system circuitry to remove the ADC offset, and it is further desirable for that mechanism to operated automatically without requiring any intervention by the user of the circuit.
Thus, there exists an unmet need for a reliable, accurate and inexpensive apparatus for effecting bidirectional communication of both analog signal information and control information across a high voltage isolation barrier, while avoiding the shortcomings of the prior art.
In one aspect, the invention provides a delta-sigma analog-to-digital converter (ADC) offset calibration system in a digital capacitive isolation system having a powered circuit on a first side of a capacitive isolation barrier and an isolated circuit on a second side of the barrier, wherein digital signals are transmitted across the isolation barrier, and wherein an ADC requiring calibration is located on the second side of the isolation barrier, the ADC offset calibration system comprising a digital integrator connected to receive an output signal from the ADC and to provide an integrated offset calibration signal; a data register connected to receive and hold the integrated offset calibration signal, the data register outputting a held offset calibration signal; a digital-to-analog converter (DAC) having an input connected to receive the held offset calibration signal and having an output providing an analog offset calibration signal; and a hybrid circuit including a signal path connecting the output of the DAC to an input of the ADC; whereby the analog offset calibration signal is connected to the input of the ADC requiring calibration.
In another aspect, the invention provides a method of performing analog-to-digital converter (ADC) offset calibration in a digital capacitive isolation system, comprising maintaining a data input signal into the ADC being calibrated at a level of zero; integrating an output signal from the ADC to provide an integrated offset calibration signal; holding the integrated offset calibration signal in a data register; converting the integrated offset calibration signal from a digital signal to an analog offset calibration signal using a digital-to-analog converter (DAC); adding the analog offset calibration signal to said data input signal, using a hybrid circuit signal path to connect the analog offset calibration signal from the DAC to an input of the ADC; and latching the integrated offset calibration signal in the data register when the output signal from the ADC becomes zero.
In a still further aspect, the invention provides a telecommunication hybrid circuit for processing an analog transmitted signal to be coupled to a communication system at a transmitter node and an analog received signal received from the communication system at a receiver node, wherein the received signal includes a portion of the transmitted signal, the hybrid circuit comprising a transmitter amplifier having an output coupled to the transmitter node; a receiver amplifier having an input coupled to the receiver node; an attenuated signal path between the input of the transmitter amplifier and a subtractive input of the receiver amplifier; and a calibration mode switch coupled between the receiver amplifier and the receiver node, the calibration mode switch alternately connecting the input of the receiver amplifier to the receiver node or to a preselected signal.